This invention relates to a method and apparatus for oxidation of ion species in aqueous solutions, including wet process phosphoric acid and leaching liquors, to provide for recovery of dissolved materials such as iron, vanadium, uranium and copper or to condition the aqueous solution for other uses. Such conditioning includes elevation of the valence state of certain dissolved ion species such as ferrous iron (Fe.sup.+2 .fwdarw.Fe.sup.+3), uranium (U.sup.+4 .fwdarw.U.sup.+6) and vanadium (V.sup.+3 .fwdarw.V.sup.+4) among others, for a multitude of purposes, such as corrosion control and metal recovery.
In this respect, phosphoric acid is commercially produced by one of two methods; "furnace" or "wet process". In the "wet process" method, the phosphate ore is contacted with a mineral acid, such as sulfuric, to extract phosphate values. In the process many other metallic compounds present in the ore are also dissolved by the acid and remain in the phosphoric solution. In the solvent extraction of some metals such as vanadium or uranium from the acid or as a reduction in acid corrosivity it is necessary to raise the valence states of these and other metals. The raising of these ion valence states is called oxidation.
The process of oxidation is physically possible using a variety of oxidants including air, ozone and other various chemical oxidants. However, in practice it may take several hours to achieve significant oxidation of these dissolved materials with air and most other chemical oxidants under normal processing conditions can add substantial costs to the recovery or conditioning process.
The present invention provides for the use of pure oxygen as an oxidant, in conjunction with specified process parameters: pressure, temperature and mixing conditions, achieving efficient oxidation of desired metals, in one embodiment, in less than 2 minutes. Accordingly, the present invention significantly reduces the oxidation process time in as much as about two orders of magnitude from prior known air or oxygen oxidation processes.
One such known process involves the recovery of uranium as a by-product from wet process phosphoric acid disclosed in U.S. Pat. No. 3,835,214 entitled Oxidative Stripping process for the Recovery of Uranium from Wet-Process Phosphoric Acid, issued to Hurst and Crouse, on Sept. 10, 1974. In the Hurst process, uranium is oxidized with an oxidizing agent which can either be oxygen or ozone gas. However, the disclosed process did not recognize the significant effect of certain process parameters and apparatus as discovered by the present invention.
A further oxidation process concerned with the recovery of uranium and vanadium from aqueous wet process phosphoric acid is disclosed in U.S. Pat. No. 3,836,476, entitled Simultaneous Recovery of Vanadium and Uranium from Oxidized Wet Process Acid, issued to Baldwin, et al on Sept. 17, 1974. The Baldwin disclosure indicates the possibility of using oxygen as the oxidizing agent, but, also fails to recognize the significance of specified reaction conditions, and significant impact regarding reduction of retention time necessary for the oxidation process.
Other patents which disclose recovery of various materials from acid solutions by oxidation include U.S. Pat. No. 2,211,119, entitled Method of Recovery of Vanadium from Acid Solutions issued to Hixson, et al. on Aug. 13, 1940; U.S. Pat. No. 2,830,872, entitled Recovery of Uranium Values from Phosphate Rock, issued to Bailes et al. on Nov. 4, 1958; and U.S. Pat. No. 2,926,992, entitled Process for Recovery of Uranium Values, issued to Stedman on Mar. 1, 1960. Although each of these disclosures discusses the recovery of certain materials from acid solutions, they are only of general interest in light of the present invention in that they fail to recognize the significant improvement achieved by the present invention over the use of conventional oxidants and process conditions.
The improved method and apparatus of the present invention therefore provides the use of pure oxygen as an oxidant at specified process parameters including pressure, temperature and mixing conditions to thereby significantly decrease the process time that is needed to oxidize dissolved materials as by-products from aqueous solutions or to condition the solution for other uses by raising the valence state of certain dissolved materials therein.